It may be possible that there are air pockets in the soil around the rootball that are drying out the roots and restricting the tree's ability to take up water. To correct this, flood the area with water and then step firmly around the top of the planting hole to help collapse any air pockets.

Depending on the kind of soil you have, while the surface may look like its draining well, if you have a base of clay covered in good topsoil, it may appear that the soil is draining but water is pooling in the newly dug soil. If you suspect this, you can try digging a hole nearby the tree to test the ability of the soil to carry away excess soil. Dig a hole as deep as the hole you planted the tree in and fill it with water. See how fast the water runs out. If it is not drained in a few hours, then there is a good possibility that the tree is sitting in an underground puddle and this is causing the problem. If this is the problem, I would recommend replanting the tree. Dig the hole twice as wide and back fill it with only good topsoil and no clay.